Æthelflæd (Old English: Æðelflæd (869 / 870–918), was the eldest daughter of King Alfred the Great of Wessex and Ealhswith, wife of Æthelred, ealdorman of Mercia, and after his death, ruler of Mercia (911–918). The Anglo-Saxon Chronicle styles her "Lady of the Mercians" (Myrcna hlæfdige).

Æthelflæd is mentioned by King Alfred's biographer Asser, who calls her the first-born child of Alfred and Ealhswith and a sister to Edward, Æthelgifu, Ælfthryth and Æthelweard. By the time he wrote, roughly about the year 890, she was already married to Æthelred, then ealdorman of Mercia.

They had one daughter, Ælfwynn.

During a sustained campaign of repeated attack between 865 and 878 the Danish Vikings overran most of the English Kingdoms such as Northumbria, Eastern Mercia, East Anglia and even threatened the very existence of Wessex. Alfred and his descendants reconquered these lands from the Danes by 937. The aid given him in this by Mercia had to be acknowledged. Instead of making the dominion of Wessex over Mercia seem like a conquest, Alfred married Æthelflæd to Æthelred of Mercia and gave his son-in-law the title Ealdorman or Earl of Mercia, thus allowing some ongoing autonomy. Since much of Western Mercia was never under the control of the Danes, and remained strong, this was a prudent move. Further prudence prevailed when the kingdoms were finally absorbed; they were not absorbed into Wessex or greater Wessex but into England. The term Anglo-Saxon thus reflects King Alfred's diplomatic integration of the Mercians Angles and the Saxons.